1. Field of the Invention
The present invention relates to radio wave communications, more particularly to reflected wave systems providing position indication for collision avoidance and with altitude determination for presence detection and landing systems.
2. Description of the Prior Art
Systems utilizing reflected wave energy to detect approach to an obstruction or other object are well-known. However, prior art systems for this purpose typically give indeterminate results when the energy is returned from both an object and a surface on which the object is disposed. Although different doppler frequencies are returned by the object and surface, the use of techniques such as highly directive antennas, a plurality of carrier frequencies, and complex signal processing such as a Fourier transform are often not practically usable to distinguish the frequencies because of time and cost constraints. This is particularly true in light aircraft landing systems where the accurate determination of distance to a landing surface or an obstruction thereon, together with the rejection of clutter, is highly desirable, but has hitherto involved expensive and complex apparatus and methods.